Influenza viruses are a significant health concern for animals and humans. The World Health Organization (WHO) estimates that every year influenza virus infects up to 1 billion people, with 3-5 million cases of severe disease and 300,000-500,000 deaths annually (Meltzer et al., 1999). The traditional approach to controlling influenza A virus is based on diagnosis, treatment and prevention through vaccination. Each of these approaches, however, has flaws (e.g., antiviral resistance, incomplete protection, and improper vaccine distribution), e.g., treating every case with antiviral drugs is not a viable option because it is often ineffective and leads to viral resistance.
Highly pathogenic avian influenza (HPAI) H5N1 viruses have spread as far as Eurasia and Africa since their first emergence in 1996. These viruses infect a range of domestic and wild avian species as well as mammals (Pollack et al., 1998; Lazzari and Stohr, 2004), and pose a pandemic threat (Allen, 2006; Anonymous, 2005; Conly and Johmston, 2004). Current prevention and treatment strategies for H5N1 virus are antiviral, vaccine-based, or involve non-pharmaceutical measures, such as patient isolation or hand sanitation (Alexander et al., 2007; Ferguson et al., 2005; Ferguson et al., 2006; Iwami et al., 2008; Lipsitch et al., 2007; Stilanakis et al., 1998). However, these approaches have flaws (Iwami et al., 2008; Lipsitch et al., 2007; Lipsitch et al., 2009; Gandon et al., 2001).
Generation of inactivated vaccines (INV) has been optimized for seasonal flu, but presents several challenges for H5N1 viruses, including: 1) continual evolution of the viruses makes predicting a vaccine strain difficult; 2) egg propagation of vaccine stock is hindered due to the high lethality of H5N1 viruses to eggs and the poultry that provide them; and 3) the six to nine month time-period required to produce INV may be too long to protect large populations during a pandemic. In addition, initial studies in mice, ferrets and phase 1 human clinical trials have demonstrated that INV and other split-virion vaccines may require higher doses of antigen than traditional INV, with more than one administration needed to provide protective immunity (Cox et al., 2004; Ehrlich et al., 2008; Wright, 2008). Live vaccines elicit both humoral and cellular immune responses. However, they are not recommended in infants, elderly, or immuno-compromised individuals because they can cause pathogenic reactions (Jefferson et al., 2005; Kunisaki and Janoff, 2009; Mostow et al., 1969; Peck, 1968). Moreover, live vaccines can revert to wild-type viruses, potentially leading to vaccine failure and disease outbreaks (Mostow et al., 1979).
Current vaccines need to be improved to overcome limited cross protection, short duration of immunity and/or lack of robust protection. For instance, a critical failure in preparation for influenza pandemics and seasonal epidemics is the absence of a universal vaccine. This is due in part to the extraordinary genetic and antigenic variation of the virus, a consequence of rapid evolution in the form of antigenic drift and shift. Indeed, influenza strains vary by 1-2% per year, and vaccines generally do not elicit protection from one year to the next, necessitating frequent vaccine updates. This diversity represents a significant challenge to the development of a broadly effective vaccine, as no single viral variant can induce immunity across observed field strains, and incorporating all circulating variants into one multivalent vaccine isn't feasible.
Multiple approaches have been studied to develop a universal influenza vaccine that could be applied to H5N1 viruses. One approach is to use conserved sequences such as the stalk region of HA or the internal NP or M1 proteins. Another approach involves consensus sequences that combine many H5N1 hemagglutinin sequences into a single gene. Of these approaches, only the consensus approach has shown partial protection against a diverse panel of H5N1 isolates. Nevertheless, a broadly effective strategy for H5N1, or other pandemic viruses, control remains elusive.